New Uses of SulfurâII - ACS Publications

10 Sulfur as an Asphalt Diluent and a Mix Filler IMANTS DEME

Downloaded by UNIV OF LIVERPOOL on December 6, 2015 | http://pubs.acs.org Publication Date: March 1, 1978 | doi: 10.1021/ba-1978-0165.ch010

Shell Canada Limited, Oakville Research Centre,P.O.Box2100, Oakville, OntarioL6J5C7,Canada

Sulfur is used currently in two types of asphalt paving proc esses. One uses a low amount of sulfur as an asphalt diluent in conventional asphalt concrete mixes. The other process, Thermopave, uses higher sulfur contents where the excess sulfur performs as a mix filler, permitting attainment of high-quality paving materials using inexpensive sands. The differences between the two processes and the roles of the sulfur in each type of paving mix are described emphasizing microscopic studies of mixes which show that sulfur is readily dispersed by aggregate shear forces during mixing in the plant pugmill. Pre-emulsification of sulfur-asphalt binders is not essential in either process. The emulsified portion of sulfur performs as an asphalt extender while any excess sulfur performs as a mix filler or stabilizer.

Τ Tarious researchers have attempted to use sulfur in asphaltic mixes * over the years; however, only recently has it become practical to do so. This is attributable partly to the development of new technology and partly to a major shift in the price of road building materials. The prices of asphalts and aggregates have escalated substantially within the last decade, whereas the price of sulfur, although cyclic, has followed a down ward trend. The decrease in sulfur cost is attributable to the increase in the available sulfur reserves associated with its removal from natural gas and the clean-up of industrial fuels and stack gases. The surplus from the latter source is expected to increase in heavily industrialized areas as environmental controls become more restrictive in the future (I). There are two reasons for considering the use of sulfur in asphalt mixes: to improve mix quality and/or to reduce mix cost. A small quantity of sulfur may be dispersed in asphalt, permitting a cost reduc tion of binder for hot-mix made with conventional graded aggregates. A number of individuals and organizations are currently involved in the 0-8412-0391-l/78/33-165-172$05.00/0 In New Uses of Sulfur—II; Bourne, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


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173

testing and commercialization of processes using sulfur as a diluent for asphalt binders: Société Nationale des Pétroles d'Aquitaine (SNPA) (2), Gulf Canada Limited (3, 4, 5), Pronk-Sulphur Development Institute of Canada (6), U.S. Bureau of Mines, Texas Transportation Institute, and D. Y. Lee (7). Some of the above cite a need for specialized equipment to disperse the sulfur in the asphalt while some cite improvement in certain mix properties such as Marshall stability, depending upon the amounts and relative proportion of sulfur to asphalt used. Most of the conventional construction equipment may be used to process, transport, and place these mixes. The Thermopave process, developed by Shell Canada Ltd. (8, 9,10, 11, 12), uses significantly greater quantities of sulfur than the sulfurdiluted process above. When liquid, the sulfur in Thermopave increases the mix workability to a point where it is placed without roller compaction. The liquid sulfur also conforms to the shape of the void spaces in the mix. When it cools, the principal action of the solidified sulfur is to key in the asphalt-coated aggregate particles. This yields high-quality mixes, even with inferior aggregates and sands. The manufacture of highquality Thermopave mixes with ungraded sands, in lieu of conventional graded aggregate hot-mix, offers a cost saving in many areas where sands are readily available and aggregates are expensive. Sulfur addition to other specified aggregate formulations permits the attainment of mixes with distinctive characteristics, applicable to a variety of specialized uses as described in Refs. 8, JO, and 12. While hot-mix plants may be adapted readily for Thermopave processing by adding a liquid sulfur supply system, specialized equipment is required for mix transport and placement. Various features of the Thermopave process are covered by patents and patent applications in a number of countries. Despite numerous publications about Thermopave and sulfur-extended binder processes, a considerable amount of confusion exists about the fundamental concepts governing these two complementary systems. This chapter, therefore, demonstrates how they are related and outlines the mix properties which are most significantly influenced by sulfur addition. Extension of Asphalt with Sulfur A number of researchers have studied the effects of treating asphalts with sulfur. The solubility of sulfur in asphalt increases with temperature but is quite low at temperatures below 149°C (the maximum safe temperature for practical mix handling operations to avoid hydrogen sulfide formation). Attempts have been made to form homogeneous dispersions of sulfur in asphalt by mixing, emulsification, and pumping action. The

In New Uses of Sulfur—II; Bourne, D.; Advances in Chemistry; American Chemical Society: Washington, DC, 1978.


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general concensus is that sulfur dispersion has a favorable action on asphalt ( 13,14,15, 2,7). Some of the beneficial properties indicated are increase in penetration value, lowering of the Fraas breaking point tem perature, lowering of the softening point temperature, improvement of the penetration index, and an increase in binder ductility. Controversy exists over the nature of the chemical reaction of sulfur with asphalt, the role of the sulfur remaining in colloidal solution/dispersion in the asphalt, and the maximum permissible sulfur concentration in the asphalt which will yield a stable long-term binder. Several investi gators, such as Kennepohl et al. (4), have shown that on a long-term basis, approximately 20% of the sulfur remains in a dissolved and/or a dispersed state as part of the binder. Figure 1, prepared from their differential scanning calorimeter data, indicates that excess sulfur, i.e., free sulfur above approximately 20 wt % , solidifies to a crystalline state, ceasing to perform as a binder. High shear during mixing is essential to obtain good sulfur dispersion and a stable binder. Certain agencies specify the use of colloid mills to achieve this. On the contrary, our experience with Thermopave and other sulfur asphalt products has been that sulfur is well dispersed by aggregate shearing action during mixing in the pugmill of a hot-mix plant. Nevertheless, laboratory and field studies were undertaken to assess the influence on sulfur dispersion of several aggregate variables, the

•

DISSOLVED SULPHUR

Π CRYSTALLINE

SULPHUR

MAXIMUM SULPHUR CONTENT REMAINING DISPERSED IN BINDER LONG - TERM

10

20

30 SULPHUR ADDED,

40 7. WT.

50

Proceedings of the Association of Asphalt Paving Technologists

Figure 1. Rehtive amounts of crystalline and dissolved sulfur in sulfur-asphalt binders as determined by differential scanning calorimetry (4)


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Table I.

175


Gradation of Materials Used in Sulfur Dispersion Study Percent Passing

ASTM

mm

y m. % in.

13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075


2

No. 4 8 16 30 50 100 200

Surface area (cm /g) 2

A Graded Aggregate 100 74

— 48 40 30 21 9 3 0

32.5

Β Crushed Stone 100 78 21 0

15.5

C Sand

D Glass Beads

100 97 92 85 5 1 0 57.2

100 1 0 34.8

mixing mode, and mixing time within the framework of formulations commonly used for sulfur-extended binder mixes and Thermopave. Sulfur Dispersion by Laboratory Mixer, The influence of the fol lowing variables on the dispersion of sulfur in asphalt was assessed in the laboratory. The mixes were prepared using 85-100 pen grade asphalt with all of the mix ingredients preheated to 140°-150°C. (1) Aggregate (for gradation see Table I): (a) Spherical glass beads (b) Sand (c) Crushed stone (d) Dense graded aggregate (2) Mixing sequence (using a small Hobart mixer, model N-50, 80 rpm, flat beater): (a) Simultaneous—all ingredients were combined in a single wet-mix cycle (b) Regular—aggregate was pre-mixed with asphalt for 30 sec followed by the addition of sulfur and mixing in a second wet-mix cycle for the various time periods described below (c) Reverse—opposite order to (2b), with the sulfur pre-mix followed by asphalt addition and mixing (3) Mixing time (after last mix ingredient added): 10, 30, 60, and 90 sec (4) Mix types: (a) Sulfur-extended binder mix—binder content, 7 wt % mix; binder composition, 1.75 wt % sulfur and 5.25 wt % asphalt; sulfur/asphalt weight ratio, 0.33 (b) Thermopave mix—asphalt content, β wt % mix; sulfur content, 12 wt % mix; sulfur/asphalt weight ratio, 2.0



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Slides for microscopic examination of the binders were prepared as follows. A glass slide and a glass rod were heated to approximately 150°C. Hot mix was pressed against the slide and then removed, leaving some binder on the slide. This was smeared with a single stroke of the glass rod to make a thin binder film of variable thickness. The slide was then examined with a high-powered optical microscope using light trans mission and adjusting polarization until the definition of the dispersed sulfur droplets was optimum. The initial examination was performed within a few minutes of mix preparation. But later examination over periods up to 48 hr showed no significant changes in the appearance of the binder. Additionally a sulfur/asphalt dispersion was prepared in a Waring blender by mixing sulfur and asphalt for 5 min at 20,000 rpm. Examined under the microscope, the supercooled dispersed sulfur globules were approximately 4 μ in diameter. This is typical of sulfur dispersions in asphalt produced by SNPA (2) and Gulf Canada Ltd. (3) using colloid mills and provided a basis for assessing the effectiveness of sulfur disper sion by the shearing action of the aggregate during mix processing. A study of the binders prepared by aggregate shearing during mixing revealed that good dispersion, with only a few sulfur globules larger than 4 ft, could be attained generally at mixing times as short as 10 sec, regardless of the order in which the constituents were combined. This is believed to result from both the low viscosity of the sulfur (easily sheared) and from the high aggregate shear during mixing. The crushed stone mix (Aggregate Β in Table I ), probably because of its lower specific surface area, was not as effective in dispersing the sulfur. In this case, examination of the binder showed a few sulfur globules considerably larger than 4 μ up to 30 sec mixing but not after 60 sec mixing. Binders from all of the other aggregate mixes showed good sulfur dispersion after mixing for 30 sec. Longer mixing times did not yield a finer disper sion of sulfur, indicating that the optimum mixing time with the last ingredient added was between 10 and 30 sec. Attainment of fine sulfur dispersions such as described above are crucial to utilization of sulfur as an asphalt diluent. Provided that they remain stable, the binders will exhibit the sulfur-extended binder proper ties described previously. A typical Thermopave mix using sand (Aggregate C in Table I) was prepared using the regular mixing sequence with 6 wt % asphalt and 12 wt % sulfur, a considerably higher amount than used in sulfur-extended binders. Examination of the binder phase on thin film portions of the microscope slides showed numerous finely dispersed sulfur particles. The predominant sulfur particle diameter was 4 μ, similar to the size observed for the sulfur-extended binders prepared in the blender. This also indi-



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cated that shear by aggregate during mixing was as effective in Thermo pave mixes as in the sulfur-extended binder mixes. The number and size of the large sulfur globules increased with the sulfur-asphalt film thickness on the microscope slide. It is postulated that all or most of the sulfur is well dispersed during mixing but that because of the presence of more sulfur than can be held in suspension by the asphalt, agglomeration of the "excess" sulfur takes place soon after mixing which solidifies on cooling and ceases to perform as an effective part of the binder. The rates of sulfur agglomeration and reaction with asphalt are not known, but their joint effect, combined with that from additional changes in the crystal structure of the free sulfur, is reflected by the rate of mix curing, as discussed later. Any reacted sulfur and the discrete sulfur particles which remain in suspension in the bitumen long term can be expected to perform as asphalt diluent in Thermopave. But no saving in asphalt content can be realized in optimum-quality Thermopave made with sands, relative to conven tional graded aggregate hot-mix. This is because sands have a higher specific surface area, as shown in Table I, and require a higher binder content to yield a binder film as thick as with dense graded aggregate mixes. For the 6 wt % asphalt - 12 wt % sulfur Thermopave formulation under consideration, assuming 20% extension of the asphalt with sulfur, the effective binder content is: 6% + (6% X 0.20) =7.2%. The remaining 10.8 wt % of sulfur performs as a mix filler. These large agglomerations of sulfur do not perform in the same way as conventional mineral fillers which are dispersed in asphalt hot-mix. The latter, at the concentrations typically used, generally improve the void filling capacity of the asphalt binder (or reduce the V M A ) without effecting such dramatic changes to mix stability as does sulfur. In summary, the laboratory studies demonstrated that aggregate shear during mixing yields 4-μ sulfur particle dispersions in asphalt, essential for long-term extension of asphalt in conventional hot-mixes. Increasing the sulfur content beyond the solubility/dispersion limit, be lieved to be approximately 20 wt % of asphalt, and use of substantially higher sulfur contents as in Thermopave resulted in coagulation of the excess sulfur and formation of large droplets which crystallized on cooling and ceased to perform as an effective part of the binder. The mixing time required to attain a fine sulfur dispersion of uniform density depends upon mixer speed and design. The laboratory scale studies performed were judged inadequate for predicting the performance of large plant pugmills, and it was judged essential to verify the findings using several typical commercial hot-mix asphalt plants.


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NEW USES OF SULFUR—Π


Sulfur Dispersion in Asphalt Plants* Sulfur-extended binder and Thermopave mixes were prepared in several batch type commercial hot-mix plants to verify the effectiveness of aggregate shear on sulfur dispersion during mixing in the pugmill. The influence of the following variables on the dispersion of sulfur was assessed. (1) Sulfur-extended binder system—binder content, 6 wt % mix; binder composition, 4.5 wt % asphalt, 1.5 wt % sulfur; sulfur/ asphalt weight ratio 0.33. (a) Aggregate—crushed stone in., 6-12 mm) was used as it has the least effective dispersive effect in the labora tory study. (b) Mixing plants—A range of batch-type models of various capacities was selected: (i) Barber-Greene capacity: 5,0001b (2,268 kg) batch size evaluated: 4,0001b (1,814 kg) (ii) Hethrington and Berner capacity: 6,0001b (2,722 kg) batch size evaluated: 4,0001b (1,814 kg) (iii) Cedarapids capacity: 10,0001b (4,536 kg) batch size evaluated: 7,000 lb ( 3,175 kg ) (c) Mixing times—liquid sulfur was added to the pugmill im mediately after asphalt. Time required to add sulfur, 5 sec. Mixing times investigated (after sulfur addition), 15, 30, 45, and 60 sec. (2) Thermopave mix (medium sand)—asphalt content, 6 wt % ; sul fur content, 13 wt % ; Plant, Cedarapids; capacity, 6000 lb (2722 kg) batch; batch size evaluated, 5000 lb (2268 kg); mixing time, 25 sec with sulfur. Microscope slides of the sulfur-asphalt binders were prepared at the plant sites. For the sulfur-extended binder mixes, the slides were photo graphed within 2 hr of preparation. For the Thermopave mix, the slides were photographed two days after preparation. Examination of thin sulfur-extended binderfilmsshowed good disper sion with most of the sulfur reduced to less than 4 μ in diameter after a 15-sec mixing time and only the odd particle of larger diameter present, as shown in Figure 2. After 30 sec of mixing with sulfur, binders from all of the plants showed, without exception, good dispersion and uniform density. Their typical appearance is shown in Figure 3. The binders mixed with sulfur for 45 and 60 sec were similar in appearance to Figure 3, demonstrating that mixing beyond an optimum time interval between 15 and 30 sec does not improve the quality of the dispersion. This verifies that aggregate shear during mixing is effective in dispers ing sulfur in commercial hot-mix plants. Addition of asphalt and sulfur to the pugmill simultaneously would permit the manufacture of sulfurextended binder mixes at the usual mixing cycle time.



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Figure 2. Appearance of sulfur particles dispersed throughout asphalt phase of sulfur-extended binder mix after 15 sec mixing in Cedarapids pugmitt. Batch size: 7000 lb.

Figure 3. Improved sulfur particle dispersion in sulfurextended binder mix after 30 sec mixing in Cedarapids pugmiU. Batch size: 7000 tb. Examination of thick sulfur-extended binder films, mixed with sulfur for 30, 45, and 60 sec, showed the presence of some larger sulfur particles. As the particle density on the slide is higher in thick films, this demonstrates that some coagulation of the sulfur particles takes place if the sulfur content is too great, relative to the amount of asphalt (sulfur/ asphalt weight ratio, 0.33). Similar evidence of coagulation of particles was obtained in thick films taken from the laboratory Waring blender dispersion for a similar blend. The dispersion in a thin sulfur-asphalt film from the Thermopave mix is shown in Figure 4. The presence of sulfur particles smaller than



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Figure 4. Thin sulfur-asphalt fim from a Thermopave mix showing variety in sulfur particle size 4-μ diameter demonstrates that the shear during mixing effectively disperses the sulfur particles. The presence of the larger particles indi cates that sulfur coagulation occurs rapidly because of the large amount of "excess" sulfur (sulfur/asphalt ratio, 2.0). The coagulation is significant in thick sulfur-asphalt films, as shown by the presence of large sulfur globules in Figure 5. Table II. Mix No.

. j Binder Content (wt%)

1 2 3 4

5 5 5 5

D

Effect of Asphalt Replacement

Binder Composition wt% Asph.

wt% Sulfur

100 80 60 50

0 20 40 50

Marshall Stability after 3 Days : (lb) (N) 1947 1540 2017 3420

(8661) (6850) (8972) (15212)

•Aggregate and asphalt Β properties appear in the Appendix. Sulfur-extended

Table III. Mix No.

. , Binder Content (wt%)

1 5 6 7

5.0 5.2 5.8 6.1

D

Effect of Asphalt Replacement

Binder Composition wt% Asph.

wt% Sulfur

100 80 60 50

0 20 40 50

Marshall Stability after 3 Days : (lb) (N) 1947 1180 1766 2858

(8661) (5249) (7856) (12713)

•Aggregate and asphalt Β properties appear in the Appendix. Sulfur-extended



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Figure 5. Thick sulfur-asphaltfilmfrom a Thermopaoe mix and large sulfur globules resulting from coagulation Special Features of Sulfur-Extended Binder Mixes A number of features distinguish sulfur-extended binder mixes from conventional hot-mix and must be considered in mix design. To illustrate this, samples have been prepared from a conventional asphalt aggregate mix and from a series of sulfur-extended binders. Information on the by Sulfur on a Weight Basis Marshall Flow after 3 Days

0

Marshall Stability after 28 Days

Marshall Flow after 28 Days

(0.01 in.)

(mm)

(lb)

(N)

(0.01 in.)

(mm)

8 7 6 8

(2.0) (1.8) (1.5) (2.0)

2080 2130 3146 3883

(9252) (9475) (13994) (17272)

8 8 7 7

(2.0) (2.0) (1.8) (1.8)

binder prepared in Waring blender.

by Sulfur on a Volume Basis Marshall Flow after 8 Days

0

Marshall Stability after 28 Days

Marshall Flow after 28 Days

(0.01 in.)

(mm)

(lb)

(N)

(0.01 in.)

(mm)

8 6 7 7

(2.0) (1.5) (1.8) (1.8)

2080 1636 2540 3366

(9252) (7273) (11298) (14973)

8 7 7 7

(2.0) (1.8) (1.8) (1.8)

binder prepared in Waring blender.


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II


design of these mixes and properties of the materials used appear in the Appendix. The data in Tables II and III indicate that the Marshall stability of a mix is lowered initially when a low amount of asphalt is replaced by sulfur (i.e., 20% ). This is attributed to softening of the binder by sulfur (14). After curing over a longer time (e.g., 28 days), the stability increases; the value for the conventional mix is surpassed by mix No. 2 but not by mix No. 5. A more detailed study of curing rates, shown in Figure 6, indicates that Marshall stability does not increase significantly after a curing period of approximately eight days. 4000

Ζ Ο

15000 £ >-

UJ

3000

00

MIX DESCRIPTION

£ 2000

£

(6%

(ASPHALT Α β * * * ο •

1000

ASPHALT 20 % A / 20 % A / 35 % A / 35 % A /

BINDER) · A6G. IN APPENOIX

CONCRETE 80 % S BINDER 80 % S BINDER 65 % S BINDER 65 % S BINDER

I)

(BLENDER) (A6G. SHEAR) (BLENDER) (A66. SHEAR)

10000 S 5000

< χ < > / oc

New Uses of SulfurâII - ACS Publications

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